None.
1. Field of the Invention
The present invention generally relates to a driver circuit with bimorph actuator. The invention specifically described is a power circuit based on a balanced capacitive loading method wherein the load itself acts as an energy storage element in the energy balance system. The invention is applicable to flight and motion control elements.
2. Related Arts
A large class of active control devices incorporate small, high-force transductive mechanisms to develop mechanical force. A bimorph actuator is a specific type of transductive mechanism composed of two transductive elements bonded to a flexible metallic element functioning as a central electrode. A bimorph functions as an actuator when transductive elements are wired so that elongation occurs along one element and contraction occurs along another. Transductive mechanisms are inherently lossless, therefore energy pumped into the device is returned except for a small portion expended producing mechanical work.
Various power circuits are known within the art to drive transductive mechanisms, while linear driver circuits are the most common. Such drivers are very inefficient in that return energy from the transductive mechanism is dissipated thermally and thereby no longer available to drive the mechanism. Some improved performance is obtained with class D implementations of the electronics, however, the issue of how to store the transient return energy remains unresolved.
A more attractive solution to reverse energy flow is a regenerative driver circuit as disclosed in U.S. Pat. No. 6,001,345 issued to Murray et al. on Jan. 4, 2000. However, Murray et al. suffers two fundamental problems. First, the invention requires a negative impedance inverter that is both quite complex to achieve and never adequately demonstrated in practice. Secondly, the invention requires a large output bypass capacitor. The capacitor value is chosen according to
RLoadCFilter greater than  greater than 1/F 
where F is the ripple frequency. The ripple current is impressed by the transients in and out during switching. This leads to a minimal requirement of the output bypass capacitor, where
Cfilter greater than  greater than Cload 
is required to achieve a xcfx893db bypass. Consequently, the power bypass capacitor quickly becomes the dominating factor in terms of mass, volume, and performance at larger loads. The result is diminished advantages in terms of efficient power handling and compact implementation of the switching section in the drive topology.
Conventional power circuits are designed to drive only one side of a transductive system. When applied to a bimorph system, the lossless nature of the transducers requires nearly all of the input energy returned and either transferred out the system as thermal energy or recovered and redirected. If recovered, the energy is typically recycled with additional input side energy to drive the other symmetric load at the output side of the circuitry. The recovery-recycle methodology as applied to symmetrically coupled systems by conventional circuits produces large peaks in the power supply ripple current. Consequently, such systems are inherently unstable.
What is required is a bimorph actuator with driver circuit capable of rapidly redirecting energy between loads in the actuator. The driver circuit with bimorph should substantially reduce peak power loading without increasing total power demand, eliminate the large bypass capacitor required in the related arts, and eliminate power supply instabilities inherent to regenerative and conventional electronics.
An object of the present invention is to provide a small, lightweight bimorph actuator system with volumetrically efficient and stable driver circuit.
To these ends, the present invention provides a regenerative class D power circuit attached to a symmetrically terminated bimorph actuator. The driver circuit incorporates a new balanced capacitive loading method using the pure reactive portion of the load itself as an energy storage element in the energy balance system. A half-bridge FET topology controls charge-discharge between two halves of a bimorph actuator causing energy to be cycled from one side of the bimorph to the other. Half-bridge averaging is externally commanded via a control module whereby an imbalance is caused producing current to flow in one desired direction only. The driver circuit causes the charge to equilibrate within the bimorph in reference to the new average control module charge. The load on the driver at any given instant is the total output load, while load on the d.c. power supply is only the real power to the load used plus any switching losses. A control module, one example being a PWM, is employed as to institute power flow between symmetric loads as seen on the output side of the circuit. Bimorph actuator is comprised of a plurality of planar members. A middle planar layer is sandwiched between two mechanically and electrically similar transductive elements thereby isolating one from the other. An outer layer is thereafter bonded to each transductive layer.
The present invention is lighter and smaller with increased efficiency over the related arts. The present invention significantly reduces the high-voltage power supply bypass capacitor representing the largest component in class D and regenerative class D circuitry. The present invention enables larger effective output filter values in a smaller package thereby increasing robustness. The present invention enables the compact, lightweight implementation for driving a high-voltage bimorph actuator. The present invention effectively enables higher switching voltage into the bimorph actuator thereby retaining the high efficiency of regenerative circuits.